1. Field of the Invention
This invention relates to a method and an apparatus for finding a distance to an object, and more particularly to a range finding method and a range finder for carrying out the method, in which a distance to an object is found in such a manner as that a reference beam of light and a reflected beam of light reflected on the object are interfered with each other, and the interference light is guided to a light receiving element.
2. Description of the Prior Art
Heretofore, there has been known a distance measuring device or range finder (Japanese Utility Model Publication No. Sho 56-15522) of the recursive optical system type as shown in FIGS. 11 and 12. In FIG. 11, 1 denotes a thin film substrate which is formed with a two-dimensional type guiding wave path. The thin film substrate 1 comprises triple-layer of thin films 4, 2 and 3. The thin film 2 has a light transmitting property. The refractive index of the thin film 2 is larger than those of the thin films 4 and 3 which are disposed at both sides thereof. A coherent light P emitted by a light source 5 is made incident to the thin film substrate 1. The coherent light P is reflected by both boundary surfaces of the thin film 2 and the thin films 3,4 and propagated through the thin film 2. The thin film substrate 1 is provided at its incident side with a collimator lens system 6. The coherent light P made incident to the thin film 2 is made into a parallel pencil of rays by the collimator lens system 6. The parallel pencil of rays is split into a reference light P.sub.1 and a measuring light P.sub.2 by a half mirror 7.
The reference light P.sub.1 is reflected by a reference mirror 10 formed on a thin film substrate 1 and returned to the half mirror system 7 again. The measuring light P.sub.2 is reflected by a measuring mirror 9 as an object and returned to the half mirror system 7. The returned measuring and reference light P.sub.1 and P.sub.2 are composed by the half mirror system 7 and guided to a measuring lens 11 as an interference light. The interference light is emitted out of the film through an outgoing prism 8. The interference light emitted from the prism 8 is dark when the difference in optical distance of the reference light P.sub.1 multiplies oddly as against .lambda./2 (.lambda. is a wavelength of the coherent light P) with respect to that of the measuring light P.sub.2. On the other hand, the interference light is bright when the difference multiplies integrally as against .lambda./2. Therefore, if the measuring mirror 9 is moved in the direction as shown by an arrow G, an interference signal based on the interference light, as shown in FIG. 13, has a bright portion A and a dark portion B alternately every time the amount moved of the measuring mirror 9 is increased by .lambda.2. Accordingly, by counting the number of the bright and dark portions A and B, the amount moved of the measuring mirror 9 can be found. Similarly, by counting the amount moved of the measuring mirror 9 from the origin, the length of the object can be measured.
In the conventional range finder, since the distance moved is measured by moving the mirror 9, there is a risk that an optical path R is cut during the movement of the mirror 9. As a result, the measurement of the distance becomes unable to be carried out.
Furthermore, in this kind of a light integration type range finder, a part of the measuring light P.sub.2 reflected by the mirror 9 is reflected by the half mirror system 7 and returned to the light source 5. Likewise, a part of the reference light P.sub.1 reflected by the reference mirror 10 is also passed through the half mirror 7 and returned to the light source 5. Due to the affection of this return light, the output of the light source 5 is fluctuated and an accurate measurement is disturbed.